Magnetic recording media are widely used in audio tapes, video tapes, computer tapes, disks and the like. Magnetic media may use thin metal layers as the recording layers, or may comprise particulate magnetic compounds as the recording layer. The latter type of recording media employs particulate materials such as ferromagnetic iron oxides, chromium oxides, ferromagnetic alloy powders and the like dispersed in binders and coated on a substrate. In general terms, magnetic recording media generally comprise a magnetic layer coated onto at least one side of a non-magnetic substrate (e.g., a film for magnetic recording tape applications).
In certain designs, the magnetic coating (or “front coating”) is formed as a single layer directly onto a non-magnetic substrate. In an effort to reduce the thickness of this magnetic recording layer, an alternative approach has been developed to form the front coating as a dual-layer construction, including a support layer (or “lower layer”) on the substrate and a reduced-thickness magnetic layer (or “upper layer”) formed directly on the support or lower layer. With this construction, the lower layer is typically non-magnetic or substantially non-magnetic, generally comprised of a non-magnetic powder and a binder. Conversely, the upper layer comprises a magnetic metal particle powder or pigment dispersed in a polymeric binder.
In addition, with magnetic recording tapes, a backside coating is typically applied to the opposing side of the non-magnetic substrate in order to improve the durability, conductivity, and tracking characteristics of the media.
It is also known in the art to calender the medium during its manufacture, e.g., to pass the medium through a series of opposed rollers before winding it into a roll, to improve surface smoothness. It is also known to heat-soak magnetic tape in wound form, after the coating and calendering processes, to “cure” the tape's coatings and increase the glass transition temperatures of the binder matrices. After the curing is complete, the tape is converted for use in cartridges. Calendering occurs at a calendering temperature of, for example, between about 90° C. and about 95° C. The calendering includes passing the substrate between opposed, generally non-compliant rolls, and optionally further includes calendering the substrate between additional opposed rolls, at least one of the additional opposed rolls being generally compliant. The calendering includes off-line calendering, and additionally includes in-line calendering, using at least one generally compliant roll, prior to the heat-curing, according to embodiments of the invention.
The single layer coating on magnetic recording media, and both layers of dual-layer magnetic recording media, generally include a granular pigment. Popular pigments are metal oxides, ferrimagnetic or ferromagnetic metal oxides, and ferromagnetic metal alloys; the material in the lower layer of the dual-layer media is generally non-magnetic, and that in the upper layer is magnetic. Different pigments have different surface properties; the metal particles often have a strongly basic surface. Recording media often utilize alpha iron oxide (α-Fe2O3) particles in the formulations; dual-layer recording media may utilize such particles in the nonmagnetic lower layer formulations, along with carbon black particles. The magnetic layer of such recording media often utilize gamma iron oxide (γ-Fe2O3), magnetite (Fe3O4), cobalt-doped iron oxides, or ferromagnetic metal or metal alloy powders, along with carbon black particles.
All front coatings or layers of magnetic recording media generally include a binder composition. The binder composition performs such functions as dispersing the particulate materials, increasing adhesion between layers and to the substrate, imparting cohesion of the particles in the layers, improving gloss and the like. As might be expected, the formulation specifics associated with the requisite upper layer, lower layer, and back coat, as well as coating of the same to an appropriate substrate are highly complex, and vary from manufacturer to manufacturer; however, most binders include such materials as thermoplastic resins. Many factors affect the performance of magnetic media, including the binder system; the lubricants; the method of forming a dispersion from the ingredients; the coating, drying and calendering conditions; the level of cleanliness around the coating head and calendering rolls; the smoothness of the tape; the number, frequency, and heights of protuberances on the magnetic surface. One measure of magnetic media performance is pulsewidth, often abbreviated as PW50. PW50 is a measurement of a signal recorded at such a low density that the transitions are isolated from one another; i.e., they do not interact or interfere with one another. The amplified, unequalized and unfiltered signal from the read head is displayed on an oscilloscope and the width along the time axis of the resulting positive and/or negative pulses halfway from the baseline to their peaks is measured. This time interval is multiplied by the tape transport speed to obtain the pulsewidth, as a distance.
It has now been discovered that using magnetic recording media having multiple layers wherein the upper magnetic layer contains certain metallic pigments in the magnetic layer of a magnetic recording medium, e.g., particle pigments having a coercivity of greater than about 2000 Oersteds (Oe), with particles having lengths of less than about 100 nanometers (nm), preferably less than 80 nm at a volume concentration of greater than about 35%, significantly narrows the PW50 characteristics of the resulting medium.